As is stated above, the present application is related to the invention disclosed in a prior U.S. Pat. No. 3,857,700. In that patent, a process is disclosed for reducing highly oxidized copper converter slags to recover copper values contained therein. The process disclosed in that patent includes the step of pumping a solid reductant into the slags with a fluid cooled metal bladed mechanical stirrer. The stirrer enables reductants which are lighter than the slag to be maintained in contact with the slags for a sufficient amount of time to react with it and reduce the slags. With the reduction, metal values separate from the slag into a matte phase from which they are easily recoverable.
The present disclosure is directed to treating slags containing nickel or copper-nickel (or either of the foregoing along with cobalt) to enable the nickel or nickel-copper and any cobalt values to be recovered.
The smelting of nickel concentrates are described in "The Winning of Nickel" by Paul Queneau, D. Van Nostrand and Company, Princeton, N.J., 1967, the teachings of which are incorporated herein by reference.
One conventional nickel concentrate smelter is Inco's Plant at Thompson, Mannitoba and is described on pages 263-267 in "The Winning of Nickel". Typically, nickel sulfide concentrates contain 7.5 percent nickel, 0.36 percent copper, 41 percent iron and the remainder is sulfur and silica.
In conventional practice, this concentrate is partially oxidized in a fluid-bed roaster, and the material is then smelted in an electric arc furnace. At Inco, an 18,000 KVA submerged arc electric furnace with six in-line electrodes is employed for smelting. In the smelting furnace, concentrate is smelted to produce a matte (see FIG. 1) which is then transferred to a converter in which the iron and most of the sulfur is removed. The silica in the feed to the electric furnace goes into the slag phase and the iron oxide slag which is generated in the matte converting operation is returned to the electric furnace to recover the metal values (nickel and copper). In the flowsheet illustrated by FIG. 1, the overall nickel recovery is 97.7 percent and the copper is approximately 97 percent. The nickel-copper matte which is produced in the converting operation is cooled and treated hydrometallurgically to recover the copper-nickel values.
One of the important considerations in this flow sheet shown in FIG. 1 is the return of the converter slag to the electric furnace. First, it contains large concentrations of magnetite and second, high concentration of nickel and copper which require its return to recover these metals. In addition, magnetite is charged to the electric furnace with the roaster calcine and the two sources of magnetite, both from the fluo-solids roaster and the converter slag return, tend to form solid refractory materials in the electric furnace which reduce the rate of smelting capacity in the furnace and cause operational difficulties.
In accordance with the present invention, it has been discovered that it is possible to improve the overall economics of recovering metals by the process illustrated in the flow sheet of FIG. 1. The use of the stirred electric furnace in accordance with the present invention for the process of the type shown in FIG. 1 is shown in FIG. 2. By utilizing the stirred electric furnace to blend a reductant into the slags, it is not necessary to return the converter slags to the smelting furnace. By following the present invention, the overall nickel recovery will increase (from 97.7 to 98.3 percent) and approximately the same copper recovery will be achieved. However, by following the present invention it is possible to increase the smelting capacity of the electric furnace by about 15 percent. This increase in capacity is a direct result of not returning the converter slags to the smelting furnace. Of course, many of the problems associated with the return of the high oxygen converter slag (magnetite) will be obviated by this process flow.
A most significant advantage of the process of the present invention is that because converter slags are not returned to the smelting furnace, less air (oxygen) enters the smelting furnace. It should be apparent that when launders are open to return slag, air is introduced into the smelting furnace. The introduction of air into the smelting furnace is undesirable for many reasons. These reasons include more volume in the off gas from the furnace to be processed and more dilute sulfur dioxide in these off gases which make their treatment more expensive.
In treating the converter slag which contains nominally 1.1 percent nickel and 0.3 percent copper in the stirred-electric furnace, a high-grade matte containing 30 percent nickel and 7 percent copper is produced; it can be returned directly to the converter for upgrading to the matte product. The total slag discharge by this modified process is approximately the same as that for the conventional Inco flow sheet.
In addition to the use of the stirred-electric furnace for treatment of the converter slag, mechanical stirrers may be installed in the electric smelting furnace with the following advantages.
a. increased smelting rate of concentrates by the dispersion of energy throughout the bath. PA1 b. production of homogeneous well-mixed slags which minimize the formation of solidified refractorty materials and enhance disengagement of matte from the slag.
Of course, it should be appreciated that the present invention has utility merely beyond those processes which employ a conventional nickel concentrate smelter. For example, the present invention can be utilized to treat concentrates which are high in copper and relatively low in nickel. The present invention can also be used to optimize flash smelting of nickel-copper concentrates. These details, however, appear below.
The following publications fairly represent the prior art.
An article is one by Pimenov, L. I. and Zyezev, L. I. entitled "Reduction Electrosmelting of Converter Slags from Nickel Production", Tsvetn. Metal. 38(1) (1965), pp. 34-36. In that process, converter slags from a nickel refinery are treated in a round, three electrode electric furnace. Converter type slags containing 0.37 percent Co, 1.03 percent Ni, 49 percent Fe, 29 percent SiO.sub.2 were charged into the furnace. After treatment, the slag contained 0.1 percent Co and 0.05 percent Ni. The matte product contained 1.6 percent Co and 5.6 percent Ni, 64 percent Fe and 24.6 percent S. Yields were 72 percent Co and 93 percent Ni. The distribution between phases were: EQU D.sub.Co = 1.6%/0.097 = 16.5 EQU D.sub.Ni = 5.6/0.05 = 112
The slag reaction time was seven hours, the energy consumption 483 kw-hr/ton.
Another patent representative of the state of the art is U.S. Pat. No. 3,542,352 by Themelis et al., entitled "Apparatus for the Continuous Smelting and Converting of Copper Concentrates to Metallic Copper." In the process disclosed in this patent, as part of continuous copper smelter, there is a slag cleaning section of the furnace which is an unbaffled open vessel. In this process, there is countercurrent flow of matte and slag.
Paper No. A74-16 presented before AIME entitled New Developments in Outokumpu Flash Smelting Method by S. U. Harkki et al. U.S. Pat. No. 3,754,891 entitled Method of Producing Iron-Poor Nickel Sulphide Matte from Sulphidic Nickel Concentrates in Suspension Smelting Thereof by Bryk et al. In recent years the trend of development in the flash smelting process has been toward high-grade matte production and as a consequence, converting has been reduced considerably. Copper matte containing about 80 percent copper and nickel matte containing more than 73 percent nickel and copper with less than 3 percent iron can be produced continuously by flash smelting without essentially increasing the copper nickel contents of the slag. This is possible by making the conditions in the lower part of the shaft slightly reducing. For further details of the foregoing see U.S. Pat. No. 3,754,891.
An article by Bryk, P. et al., "Flash Smelting of Copper Concentrates", AIME, February 1958, discloses a process wherein copper in flash smelter slags is recovered by holding the slag in an electric furnace for several hours. Lime and coke are added, and copper settles into a matte. The furnace is quiescent and there is no agitation to enhance the extraction rate. Copper is reduced to 0.2 to 0.6 percent by controlling the reduction of FeO in the slag. The energy consumption is 130 kw hr/ton slag.
It is a fact that the method most commonly used for slag cleaning is treatment in electric furnace and slag flotation. This does not involve a reduction scheme but merely employs physical separation of a matte from a slag. The selection of the cleaning process for copper depends upon local circumstance, but the electric furnace treatment is, however, the best prior art method for the efficient economical recovery of nickel.
Recirculation of converter slags to electric furnaces causes certain disadvantages. Impurities such as lead, nickel and antimony are reduced together with copper, making it more difficult to process the impure material. Another disadvantage is the risk of magnetite-build up on the bottom of the electric furnace due to the high magnetite content of the converter slag. In shoft, using electric furnaces to treat converter slags has many disadvantages which are overcome by treatment in accordance with the present invention.
In short, in the most widely used prior art processes, nickel or nickel-copper is recovered from slags by either returning the slag to the reverberatory furnace to allow the nickel or nickel-copper to settle; or cooling, grinding and floating the slag; or settling the nickel or nickel-copper in an electric furnace. Each of the foregoing methods suffers from one or more deficiencies which are significantly reduced in the process of the present invention.